1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor.
2. Related Background Art
In the conventional color copying apparatus equipped with a transfer drum or an intermediate transfer member, only one image is formed on such transfer drum or intermediate transfer member in case of output of a large-sized sheet such as of A3 size, but, in case of a half-sized (A4 size) sheet, there is executed a two image formation mode (two image retention mode) for forming two images in succession on the transfer drum or on the intermediate transfer member in order to increase the throughput. In the half size image formation, such mode allows to achieve a doubled productivity.
In such image forming apparatus, the two image formation mode can usually be executed only in case of forming plural outputs from a same original in the color image formation. However, in the monochromatic output with the above-described image forming apparatus, since the transfer process is completed by one step for black color, the throughput of monochromatic image formation can be improved by scanning two originals in succession, instead of an original, and forming two images of such two originals on the transfer drum or the intermediate transfer member.
Most of the conventional color copying apparatus similar in configuration to the above-described image forming apparatus utilizes the two image formation mode in the monochromatic image formation, in order to increase the productivity.
FIGS. 26, 27 are block diagrams showing the connection of a reader unit in the conventional image forming apparatus.
In these drawings, there are shown an image reading device (reader unit) 2501 for reading an original; a host computer 2104; a printer controller 2103 for controlling the entire printer; a video I/F 2101 for executing communication between a printer engine 2102 and the printer controller 2103; a printer engine 2102 for recording transmitted image data on a recording sheet; a network 2105; a controller 2602 for transmitting print data, transmitted from the host computer 2104, to the printer engine 2102, also controlling the entire printer and executing communication with the reader unit 2501 through an I/F 2601; and a selector unit 2603 for selectively controlling the transmission of the image data from the controller 2602 or the reader unit 2501 to the printer engine 2102.
In the conventional image forming apparatus described above, the timing of image formation is determined by the function timing of the image receiving member such as the transfer drum or the intermediate transfer member, so that the replacement of the original by an original feeding device equipped in the reader unit has to be completed within a predetermined replacement time. In case the replacement of the original cannot be completed within the predetermined replacement time, the image formation is started before the replacement of the original (transportation of the original on the original table) is completed, so that proper image formation cannot be achieved.
However, the speed of original replacement is significantly affected by the paper quality of the original or the state thereof, and the time required for the original replacement may become significantly longer than the normal replacement time, depending on the quality or state of the original. If the replacement time exceeds that required for the two image formation mode, there may result a situation where the output image for the replaced original becomes distorted or the image output cannot be achieved even after retry because the situation is misjudged as an original jam state.